Sputtering, alternatively called physical vapor deposition (PVD), is the most prevalent method of depositing layers of metals and related materials in the fabrication of semiconductor integrated circuits. One application of sputtering is to deposit barrier and seed layers associated with a via structure illustrated in cross-section in FIG. 1. A conductive feature 10 is formed in the surface of a lower dielectric layer 12, typically formed of silicon oxide or other silicate glass, perhaps doped to have a low dielectric constant. An upper dielectric layer 14 is deposited over the lower dielectric layer 12, and a via hole 16 is etched through the upper dielectric layer 14 in the area of the conductive feature 12. In modern circuits, the via hole 16 may have an aspect ratio measuring the depth to minimum width of 4:1 or even greater. Metal will eventually be filled into the via hole 16 to provide a vertical electrical interconnections between a lower wiring level including the conductive feature 10 and an upper wiring level formed on or in the top surface of the upper dielectric layer 14. A simple straight via 16 is illustrated. In dual damascene, the simple via 16 is replaced by a narrow via in the lower part of the dielectric layer 14 connected to a wider trench in the upper part which extends horizontally a considerable distance to form a horizontal interconnect in the upper wiring layer.
Prior to via metallization, a liner layer 20 is deposited over the top surface of the dielectric layer 14 and on the bottom wall and side walls of the via hole. The liner layer 14 performs several functions including a barrier to diffusion between the via metal and the oxide dielectric, an adhesion layer between the oxide and metal, and a seed or nucleation layer for after deposited metal. Although aluminum was the dominant metallization in the past, copper in a dual-damascene structure is beginning to dominate advanced integrated circuits because of its lower electrical resistivity and electromigration and the ability to fill the via hole 16 with copper using of electro-chemical plating (ECP). In the case of copper, the conductive feature 10 is typically the trench portion of a dual-damascene metallization. The liner layer 20 for copper typically includes a barrier layer of tantalum nitride (TaN), an adhesion layer of Ta. A thin copper seed layer both nucleates the ECP copper and serves as an electrode for the electro-chemical process. Chemical vapor deposition (CVD) or its improvement of atomic layer deposition (ALD) may be used for some of the layers. Both techniques tend to coat conformal layers in high aspect-ratio holes, and ALD can coat very thin layers of compounds. However, sputtering is typically preferred because of its economy and good film quality if several inherent problems can be overcome. Sidewall coverage is generally poor and produces thin sidewall portions 22 deep inside the hole 16. Sputtering tends to form overhangs 24 at the top of the hole 16, which at a minimum increase the effective aspect ratio for thereafter coating into the hole 16 and at worst bridge over the top of the hole 16, preventing any further deposition into the hole 16. Various techniques incorporating electrically biasing the wafer can be used to reduce the overhangs 24 and to increase the sidewall coverage. These techniques tend to enhance the bottom coverage, as represented by a thicker bottom portion 26. However, the bottom portion 26 stands in the conductive path to the lower conductive feature. Tantalum, although a metal, has a somewhat high electrical resistivity. Tantalum nitride is significantly resistive. As a result, it is desired to etch away the bottom portion 26. On the other hand, etching of the overhangs 24 should not remove underlying barrier layers.
In the parent International Application, Ding et al. have addressed these numerous and conflicting requirements by use of a sputter reactor schematically represented in FIG. 2 which is capable of depositing both Ta and TaN. Side walls 30 of a vacuum chamber are arranged about a central axis 32 of the reactor. A tantalum target 34 is supported on and vacuum sealed to the chamber 30 through an annular isolator 36. A pedestal 38 holds a wafer 40 to be sputter processed in opposition to the target 34 along the central axis 32. A vacuum pump system 42 is capable of pumping the chamber 30 to a pressure as low as 10−8 Torr. However, argon working gas is supplied from a gas source 44 through a mass flow controller 46 to a pressure typically in the range of 0.1 to 10 milliTorr. A selective DC power supply 48 negatively biases the target 34 with respect to the grounded metal chamber 30 or its unillustrated shields to discharge the working gas into a plasma. The negative target bias attracts the positive argon ions to the target 34, and the energetic ions sputter tantalum atoms from the target 34. Some of the tantalum ions strike the wafer 40 and deposit a layer of tantalum on it. In some parts of the process, nitrogen gas is supplied into the chamber 30 from a gas source 50 through its mass flow controller 52. The nitrogen reacts with the sputtered tantalum to form tantalum nitride on the wafer 40 in a process called reactive ion sputtering.
The density of the plasma adjacent the target 34 is increased by a small unbalanced nested magnetron 56 placed in back of the target. Fu describes such a magnetron in U.S. Pat. No. 6,183,614. It includes an inner pole 62 of one magnetic polarity surrounded by an annular outer pole 64 of the opposite polarity, both supported on and magnetically coupled by a magnetic yoke. Horizontal components of the magnetic field in front of the target 34 trap electrons and increase the plasma density and hence the sputtering rate. The small area of the magnetron 60 concentrates the target sputtering power in the area adjacent the magnetron 60, again increasing the plasma density. The magnetron 56 may have various shapes including circular, oval, triangular, and racetrack. To provide uniform sputtering, the magnetron 60 is supported on and rotated about the central axis 32 by a rotary drive shaft 68. The total magnetic intensity of the outer pole 64, that is, the magnetic flux integrated across its face, is significantly greater than that of the inner pole 62 causing the magnetron 60 to be unbalanced. The ratio is at least 1.5 and preferably greater than 2.0. The unbalance causes magnetic components to project from the outer pole 64 towards the wafer 40, both confining the plasma and guiding any tantalum ions to the wafer 40.
If sufficient power density is applied to the target 34, the high-density plasma region beneath the magnetron 60 ionizes a significant fraction of the sputtered tantalum atom. The tantalum ions may be attracted back to the target 34 to cause further sputtering in an effect called self-ionized plasma (SIP) sputtering. As a result, the argon sputtering gas becomes less important in supporting the plasma, and the argon pressure may be reduced. In some situations with copper sputtering, the SIP plasma is self-sustained and the argon supply can be cut off.
A band-shaped RF coil 70 larger than the wafer 40 and having two distinct ends is positioned inside the chamber 30 and its unillustrated shields in the lower half or third of the processing space between the target 34 and wafer 40. In one embodiment, the coil 70 is made of the same material as the target, that is, tantalum in the example being discussed. Further, it has a tubular shape along the central axis 32 with an aspect ratio of axial length to radial thickness of typically at least four. This composition and shape allow the coil 70 in one mode of operation to act as a second sputtering target. A DC power supply 72 and a RF power supply 74 are coupled through unillustrated coupling and isolation circuitry to allow the coil 70 to be independently DC biased or to inductively couple RF energy into coil 70 or a combination of the two. The RF power is grounded on one end of the coil 70 through a capacitor 76, which however DC isolates the coil 70 according to the DC power supply 72. The figure does not illustrate the relative positions of the power supplies 72, 74 and the ground on the coil 70. It is preferred that the coil extend nearly 360° in a plane perpendicular to the center axis 32 so that its ends are separated by a minimal distance, for example by less than 25° about the center axis 32. One of these ends is powered; the other, grounded.
When the coil 70 is negatively biased, it attracts the argon ions to sputter tantalum from the coil 70. When the coil 70 is driven by RF power it generates an axial RF magnetic field which induces an azimuthal electric field to induce a plasma region in the lower portion of the chamber 30. That is, the secondary plasma source creates a disk-shaped region of argon ions close to the wafer. Another RF power supply is coupled through a capacitive coupling circuit 80 to the pedestal electrode 38, which induces a negative DC self-bias at the edge of the adjacent plasma. As a result, the argon ions in the secondary plasma source, as well as any from the top magnetron/target source, are accelerated to the wafer 40 and sputter etch it. Because of the anisotropy produced by the acceleration, the energetic ions reach to the bottom of the via holes and are effective at selectively etching the bottom portion 26 relative to the sidewall portion 22.
Although the illustrated reactor is capable of many modes of the operation, two extreme modes are possible. In a deposition mode, the RF power to the coil 70 is turned off. Significant DC power is applied to the target 34. Because of the self-ionized plasma, the argon pressure may be reduced to reduce any argon ion sputter etching of the wafer 40. If desired, the coil 70 may be DC biased to act as a secondary target. This mode primarily deposits tantalum with minimal sputter etching of the wafer if any. On the other hand, in an etch mode, the two DC target powers 46, 72 are turned off so essentially no tantalum is sputtered. Further, the RF current to the coil 70 is increased and the RF bias supply 78 DC self-biases the wafer 70. As a result, little tantalum is deposited and the argon ions from the second plasma source sputter etch the wafer 40. A combination of simultaneous deposition and etching can be achieved by utilizing all the power supplies 46, 72, 74, 78.
However, the etching performed by such an apparatus has been found to be very non-uniform across the diameter of the wafer. Two lines shown in the graph of FIG. 3 represent experimentally observed etch rates for 800 W and 450 W of bias power applied to the pedestal. The etch rate is highest near the center of the wafer and falls off by approximately 40% towards the wafer edge. While the sputter reactor of FIG. 2 demonstrates acceptable uniformity of sputter deposition, the sputter etch uniformity needs improvement.
The coil 70 needs to be supported inside not only the electrically grounded chamber walls 30 but also inside the grounded sputtering shields used not only to protect the walls from deposition but also to act as an anode in opposition to the cathode target 34. A simple, easily serviceable mechanical system is need to support the coil and provide electrical connections to it. A further problem, particularly with the recently developed 300 mm chambers, is that the size of the chamber needs to be minimized to reduce the foot print of the reactor in valuable clean room space.